612 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



removed from milk the skimmed milk is less opaque and white, while 

 the edges in contact with the vessel have a distinct bluish tint. 



The specific gravity of milk varies from 1018 to 1040. Cows' milk, 

 when pure, may vary between sp. gr. 1028 and 1034, usually increasing 

 from the first to the eighth month of lactation from 1031 to 1039. 



In composition milk consists of solids partly dissolved and partly 

 suspended in a fluid plasma, the amount varying from 12 to 13 per cent. 

 Of these solids, 3 to 6 per cent, is represented by the fats, 9.25 per cent, 

 by the other solids. 



In composition, milk is composed of 85 to 90 per cent, water, 

 casein, albumen, fat, milk-sugar, lecithin, and salts, with carbon dioxide, 



Fig. 259.— Microscopic Appearances op Milk, I ; Cream, II ; Butter, HI; Colos- 

 trum OF Mare, IV; and Colostrum of Cow, V. (Thanhoffer.) 



oxygen and nitrogen gases, urea, and various accidental constituents, 

 such as lactic acid after milk fermentation, hsematin, bile coloring- 

 matters, and mucin. It often serves to eliminate various substances such 

 as drugs, among which may be mentioned potassium iodide, iodine, salts 

 of various metals, the oil of garlic, and various other bodies. When 

 filtered through animal membrane or porous-clay filters, the milk-plasma 

 is obtained as a clear, slightly opalescent fluid, which contains casein, 

 serum-albumen, peptone, milk-sugar, salts — in fact, all the constituents 

 of milk, with the exception of the oil-globules and a considerable portion 

 of the casein, the amount of the latter which is kept back being greatly 

 increased when a fresh animal membrane is used as a filter. 



